A circulator is often used with other optical devices to achieve certain optical functions. For example, a circulator can be used with a Brag Grating to extract an optical signal with a particular wavelength from a Wavelength Division Multiplexing (“WDM”) optical signal. FIG. 1 shows a four-port circulator 444 with four ports 1, 2, 3, and 4. An optical signal entering port 1 exits from port 2, while an optical signal entering port 2 exits from port 3, and an optical signal entering port 3 exits from port 4.